A known vacuum brake booster is disclosed in a Japanese Patent Application laid-open publication No. 2007-22435. The disclosed vacuum brake booster is provided with a housing, a movable wall which divides the inside of the housing into a front vacuum chamber and a rear variable pressure chamber, and a valve body which is fixed to the movable wall. The valve body has an axial hole in which a plunger, a valve mechanism, a reaction member and an output member are mounted. The plunger is movable back and forth relative to the valve body. The valve mechanism includes a vacuum valve which establishes or interrupts a communication between the vacuum chamber and the variable pressure chamber in response to the back and forth movement of the plunger relative to the valve body and an air valve which establishes or interrupts a communication between the variable pressure chamber and an atmospheric air in response to the back and forth movement of the plunger. The reaction member can engage with a front end portion of the plunger and a front end portion of the valve body at its rear surface. The output member engages with a front surface of the reaction member at its rear end portion and is axially movable relative to the valve body. A vacuum passage is formed in the valve body and is communicated to the vacuum chamber at its front end. An axial passage is formed in the valve body and is communicated to the atmospheric air at its rear end portion. A radial passage is formed in a portion of the valve body in which the vacuum passage is not formed. The radial passage is communicated to the variable pressure chamber at its radial outer end and is communicated to the axial passage at its radial inner side. When the plunger is axially moved forward from its initial position relative to the valve body, the vacuum closes an arc-like valve seat portion which is formed on a rear end portion of the vacuum passage and the communication between the vacuum chamber and the variable pressure chamber is interrupted. Then, the air valve opens and the variable pressure chamber is communicated to the atmospheric air. Thereby, atmospheric air flows in the variable pressure chamber through the axial passage and the radial passage. When the plunger is axially moved rearward from the forward movement position to the initial position, the air valve closes and the communication between the variable pressure chamber and the atmospheric air is interrupted. Then, the vacuum valve opens and the variable pressure chamber is communicated to the vacuum chamber through the radial passage, the axial passage and the vacuum passage.
In the above-described vacuum brake booster, the flow of the atmospheric air flowing in the portion which the axial passage and the radial passage are connected (intersecting portion) is made smooth and the flow of the atmospheric air is prevented from becoming a turbulent flow. Therefore, the above-described vacuum brake booster can prevent from generating a response lag of the operation and from generating an operation noise due to the turbulent flow of the atmospheric air at the portion connected between the axial passage and the radial passage.
In the above-described vacuum brake booster, however, a structure for making the flow of the atmospheric air in the axial passage smooth is not adopted. As a result, the flow of the atmospheric after passing the air valve under the opening condition becomes a turbulent flow and thereby the response lag of the operation of the vacuum brake booster and the operation noise are generated.